Rough Shifts

99 3.5 engine, 77k mi. I never had a transmission problem until I had the trans. fluid and filter replaced. (Yeah, yeah, if it ain't broke, don't fix it.)Immediately thereafter I noticed a very rough shift through the gears. Ck'd the fluid level and found it very low. Mechanic said there is no leak, might have been an air bubble giving a false full reading after the fluid change. Added fluid. Now the car shifts fine when cold, gets progressively rougher in stop and go traffic at full temp. (exactly half way on the gauge) . When the guage is where it normally is, not quite half way up, the trans. shifts fine. Any ideas?

I would ask your mechanic to do the transmission change again. Ask if he will split the cost with you.